Field of the Invention
The present invention relates to a sheet processing apparatus processing a sheet and to an image forming apparatus.
Description of the Related Art
Hitherto, there is an image forming apparatus, such as a copier, a laser beam printer, a facsimile, and a multi-function printer, including a sheet processing apparatus performing a binding process or the like to a sheet on which an image has been formed. In a case of binding a sheet bundle by the sheet processing apparatus, such image forming apparatus is configured to bind the sheet bundle by using a metallic staple in general. The stapling process using such staple is adopted in many sheet processing apparatuses because it enables to steadily bind a plurality of output sheets at a position specified by a user.
However, although it is possible to steadily bind the sheet bundle by the stapling process using the metallic staple, it is necessary to use a dedicated tool in order to release the bundle once bound by the staple. It is also necessary to remove the staple in shredding the stapled sheet or in recycling the stapled sheet bundle. That is, the sheet and the staple must be separately collected.
Therefore, among the conventional sheet processing apparatuses, there is proposed one that places an importance on recyclability and is configured to bind a sheet without using a staple. As such sheet processing apparatus, there is one performing a binding process on a sheet bundle by a binding portion having convex upper teeth and concave lower teeth as disclosed in Japanese Patent Application Laid-open No. 2011-201653 for example.
After bundling and aligning sheets, the sheet processing apparatus binds the sheet bundle by entangling fibers of the overlapping sheets of the sheet bundle by engaging the lower and upper teeth of the binding portion to form concavities and convexities in a thickness direction at a part of the sheet bundle. That is, this sheet processing apparatus binds the fibrous sheets without using a staple. It is noted that the way of binding the bundle of the fibrous sheets without using the staple will be referred to as a stapleless binding method′ hereinafter.
The conventional sheet processing apparatus includes the lower teeth mounted to one end portion of a fixed lower arm and the upper teeth mounted to one end portion of an upper arm supported swingably in a vertical direction and binds the sheet bundle by engaging the lower teeth with the upper teeth by swinging the upper arm. Here, the lower and upper teeth are disposed such that a teeth arrangement direction is in parallel with a turning shaft of the upper arm. A binding direction on the sheet bundle also runs in parallel with or vertically to an edge side of the sheet bundle.
By the way, in the case when the teeth arrangement direction of the lower and upper teeth are disposed in parallel with the turning shaft of the upper arm, all teeth of the lower and upper teeth abut against the sheets simultaneously in clamping the sheets when the upper arm is swung because distances between the lower and upper teeth changes uniformly. In this case, a total load F required to bind by the lower and upper teeth is n×f, where f is a load required for a set of teeth of the lower and upper teeth to bind the sheets and n is a number of teeth of the lower and upper teeth.
Here, because strength and size of each component of the binding portion are determined by the total load F, the greater the total load F, the more the required strength increases, the more the device is enlarged, and along with that, the more the cost of the device increases. Meanwhile, because a greater number of teeth increases a fastening force of the sheet bundle, the number of teeth of the lower and upper teeth may be increased to enhance the fastening force of the sheet bundle. However, if the number of teeth is increased, the total load required for binding the sheet bundle increases. Thus, the cost increases if one tries to enhance the force fastening the sheet bundle, and if one tries to cut the cost, the sheet bundle cannot be bound by strong fastening force.